1. Field of the Invention
The present invention relates generally to the manufacture of steel-bodied bits used in the drilling of oil and gas wells. More specifically, the present invention relates to bit designs and to processes for casting bits having complex configurations, in different sizes and with modified configurations, without the use of complex, reusable pattern molds.
2. Description of the Prior Art
A basic process for manufacturing a steel bit is to machine the bit from a solid billet of steel into the desired final bit form. The basic process is improved upon by using a steel casting that has already been cast into a form approximating the final bit form, permitting a substantial reduction in machining. The procedure is complicated by the addition of the metal casting step, but the overall savings in time and cost are more than offset by the use of castings.
It is common to cast the bit body with excessive material to permit the machining of alternative design features in a particular bit. In these situations, the superfluous material that is left on the casting to provide the option of including a particular feature is machined off if the bit is to be manufactured without the additional feature. The inherent trade-off is that the more universal a casting becomes, the greater the additional machining required to remove the extra material in the casting when a bit not employing the additional feature or features is to be manufactured. This problem may be overcome to a degree by employing a specialized casting that closely resembles the final form of only a specific model bit. However, a trade-off is again necessary because the specific casting cannot be machined to include features that would be created in material not present in the casting. In normal production, a compromise is made between the flexibility allowed by machining a casting into several different end forms and the cost of having a special casting made for each of the different design forms.
In the field of bit manufacture, it is common to require relatively specialized bit sizes, shapes, and designs to meet a particular application. Frequently, the bit is built to a customer's specific requirement. The greater the diversity of design and size requirements of the particular bit, the more closely the universal casting resembles a basic steel billet. In the past, there has been a constant need to compromise between increasing efficiency and reducing the cost of production runs of a specific bit design and maintaining the capability of providing a variation in the bit size or bit design for a relatively small production run or even a single custom-made bit design.
It is known in the field of metal casting that a relatively complex physical configuration may be cast in a process in which the mold is sacrificed with each casting. One such process, referred to generally as a "lost foam" process, employs an expendable plastic pattern in an expendable mold. In a variation of this process, foam patterns are produced in a reusable metal mold or die where large numbers of identical objects are to be cast using a lost foam process. Each of the patterns produced in the die may be used in a process in which the plastic pattern is covered with a hardenable fluid material to form a shell mold. The materials and steps used in a specific process of this type are more fully described in U.S. Pat. No. 4,660,623. Once the shell mold has hardened, the foam pattern is removed through a chemical or heating process. The hardened shell is then filled with molten steel to produce the casting. Once cooled, the shell mold is broken away from the casting. The step of sacrificing a plastic pattern and an expendable shell mold is repeated for each of the metal castings.
The described lost foam procedure typically follows a multistep process, the first of which is to fabricate a positive model of the object to be cast. This model is then employed to form a reusable metal negative pattern mold or die. Depending on the complexity of the pattern to be produced by the pattern mold, the pattern mold may require multiple separable components to provide a mold that can be released from the pattern. Once the pattern mold is completed, large numbers of patterns may be produced using the mold. Any change in the design of the object to be cast requires a change in the pattern mold.
The construction of a metal pattern mold for a complex shape, such as a drill bit having a complex form that cannot be removed from a two-piece mold is time-consuming and expensive. In a typical situation that requires the building of a pattern mold for a steel-bodied bit design, fabrication of a suitable metal mold for the foam pattern may require several weeks and may cost as much as $50,000 or more. Any variation in the bit design requires a modification or fabrication of a new pattern mold with an associated time loss and expense. Accordingly, the usual procedure of employing an intermediate metal pattern mold to provide the foam patterns used in the lost foam casting process is undesirable for use in the fabrication of complex steel-bodied bits.
U.S. Pat. No. 5,197,527 describes a process in which a foam block workpiece is machined into a lost foam pattern for use in a full mold casting. The system is directed toward a process in which multiple machining stations are employed so that optimum efficiency is realized in a process where a large number of identical patterns are being fabricated. The patented procedure describes a rectilinear, three-axis machine that is positioned below the workpiece to shape the surface of the workpiece in machine movements along the three standard, mutually perpendicular axes. While the work pieces are being machined, it may be appreciated that the system could be improved with the use of cast patterns since the patterns produced in the described machining process are also appropriate for use in an intermediate pattern molding process. This observation results from the fact that patterns formed using only three-axis movement have non-complex configurations that allow them to be extracted from relatively simple two-piece molds. Variations in simple, non-complex designs of the type illustrated in the prior art process can also be easily achieved by simple modifications to the pattern mold. The described patented system is also well suited for a process in which a large number of identical items are to be cast.
U.S. Pat. No. 4,423,646 describes a process for producing a rotary drill bit in which a casting technique using a plastic foam is used to produce steel bit bodies. Foam is molded in a shape substantially identical to that of the drill bit body, and cutting members are mounted on the foam form. The foam form may be machined to produce additional bit features. The assembly is then coated with a hardenable mold material to form a mold body. The foam is burned out of the hardened mold to leave a mold cavity, and molten steel is poured into the mold cavity. After the steel has cooled, the mold is removed from the completed bit by a chemical treatment. Production of the desired foam pattern is thus seen to require a two-step process, including molding and machining.
While it is recognized that a complex pattern that may not be easily molded may be machined from a block of pattern material, the limitations of a machining process are brought to each pattern made in the process. Machining a pattern, rather than molding it in a complex die or mold, does not eliminate the problems of cost and time expenditures.
The machining of patterns permits any desired number of identical patterns to be fabricated by a properly programmed numerically controlled machine. However, as with any machining program, the greater the complexity of the machined part, the greater the time and expense required to fabricate the part. Curved parts are particularly time-consuming since they typically require a large number of machining passes to create a smooth curvature.
Conventional spiral blade drill bits have a continuously curving blade end that is used to mount cutting elements in a spiral configuration. Machining the curved, spiral blades into a steel casting is very difficult and time-consuming. Machining plastic into the curved shapes is also time-consuming and, because each pattern must be separately fabricated, the time loss for a production run is multiplied by the number of patterns being machined. Accordingly, any design change that reduces the machining complexity can provide significant time and cost savings, whether it be for steel or plastic.
The machining of plastic patterns also makes possible the creation of bit designs that would not be practical if the design were to be machined from steel castings. Thus, a surface that might require an hour to form on a steel bit body may require only a few minutes to machine onto a plastic pattern. This capability can make practical the creation of cast bit features that would be impractical if they were to be machined directly on the steel bit.
One problem encountered in the typical fabrication of a steel-bodied bit derives from the welded-on application of hardfacing to the gauge faces of the bit. If the material is not properly applied, it may make the bit over- or undersized, or it may create rough edges that grab the face of the wellbore. In either case, the bit must be reworked to correct the defect.
Conventional steel bits also employ a layer of hardfacing that extends from blade edge to blade edge across the face of the blade. If the hardfacing is not properly applied, the hardfacing layer may form a sharp edge as the bit body wears away from its contact with the harder hardfacing material. The resulting edge of hardfacing material can gauge the wellbore wall and create bit vibration and other undesired drilling actions. Accordingly, it will be appreciated that the proper application of hardfacing to the correct area of the bit can be critically important to proper bit operation.